The present embodiments relate to a magnetic resonance system and a method for carrying out magnetic resonance measurements in an intraoral region.
Diseases of the teeth and of the periodontium (e.g., periodontitis or caries) are diagnosed using X-ray based imaging methods. X-ray technologies employed for this purpose range from conventional X-ray methods, through digital X-ray methods in projection mode to new types of 3D X-ray methods.
In 3D diagnostic radiology, digital volume diagnostics (DVT) systems are on the market. Complete jaw regions may be radiographed or, additionally, high-resolution 3D images of tooth and jaw regions may be produced. The radiation used in digital systems is reduced in comparison with conventional diagnostics, and an image is immediately available. The DVT systems pet nit a type of X-ray computer tomography of the teeth and of the visceral cranium at a high resolution and positional accuracy. Such DVT diagnostic systems are, however, very complex and expensive, which provides that 3D diagnostic radiology is used only in a few indications. There is therefore a need for new imaging systems and methods in the jaw region.
For many diseases of the teeth or of the periodontium, a magnetic resonance tomography (MRT) examination is a good alternative to the previous methods such as, for example, diagnostic radiology, since MRT is free from ionizing radiation and also enables contrast in soft tissue to be better represented.
In a magnetic resonance device, the body to be examined may be subjected to a relatively high basic magnetic field of 3 or 7 tesla, for example, with the aid of a basic magnetic field system. In addition, a magnetic field gradient is created with the aid of a gradient system. Using a high-frequency send system, high-frequency magnetic resonance excitation signals (HF signals) are sent out by suitable antennas. This may result in the nuclear spins of certain atoms resonantly excited by this high-frequency field being tilted by a defined flip angle with respect to the magnetic field lines of the basic magnetic field. When the nuclear spins are relaxed, high-frequency signals (e.g., magnetic resonance signals) are emitted. The magnetic resonance signals are received by suitable receive antennas and processed further. From the “raw data” thus acquired, the desired magnetic resonance image data (MR image data) may be reconstructed. A position encoding is effected by switching appropriate magnetic field gradients in the different spatial directions at precisely specified times (e.g., when sending out the HF signals and/or when receiving the magnetic resonance signals). The high-frequency signals for the nuclear spin magnetization may be sent out by a “whole body coil” or “body coil” permanently incorporated in the magnetic resonance tomograph. A typical construction for this is a cage antenna (e.g., a birdcage antenna) that consists of a plurality of send rods that are arranged running parallel to the longitudinal axis around a patient chamber of the tomograph, in which an object under examination (e.g., a patient) is situated for the examination. On a face side the antenna, rods are each connected to one another in the form of a ring. In order to receive the magnetic resonance signals, local coils that are attached close to or directly on the body of the object under examination may be used. Such local coils include one or more conductor loops.
Non-bony tissue structures may be represented in position-related and tissue-specific fashion by MRT. Areas of application for MRT in the jaw region existing hitherto lie principally in the examination of the temporomandibular joints or of the floor of the mouth. Intraoral local coils for a dental MRT that enable an anatomical representation of the jaw region and also the representation of dental diseases are thus known.
MRT diagnostics have, however, hitherto not yet become established in tooth imaging or imaging in the jaw region because only very inexpensive MR systems are suitable in this area for economic reasons. However, these cannot deliver a sufficient level of positional accuracy on account of the frequently occurring inhomogeneity of the basic magnetic field, non-linearities of the gradients, eddy currents and other disruptive influences. This is due to the distortion effects that are well known in MRT diagnostics. Thus, for example, numerous methods for distortion correction that are based on calculating the deviations of the system from the ideal state and correcting the images accordingly are known with regard to MRT. The applications DE 10 337 241 A1 and DE 10 2006 033 248 A1 are cited as examples of such distortion correction methods.
This situation is further aggravated by the fact that the object under examination (e.g., the actual patient) changes electromagnetic fields (therefore also the HF signals) in the system, and thus, a distortion caused by the object under examination or a patient is an additional disruptive factor to the normal distortion, based, for example, on an inhomogeneity of the magnetic fields.
Such distortions result in a poor positional accuracy that is particularly disruptive in dental medicine, since the images are intended to be used, for example, for planning implants and prostheses. In this situation, even small deviations are extremely critical because the implant or prosthesis does not subsequently fit.
High demands on the basic field homogeneity and gradient linearity and also special measures for homogenizing the fields of the HF signals do, however, make MR systems rather expensive and are barely able to compensate for distortion effects caused by the object under examination or a patient.